Ribosomal proteins in cell proliferation and apoptosis.

Ribosomal proteins have the complex task of coordinating protein biosynthesis to maintain cell homeostasis and survival. Recent evidence suggests that a number of ribosomal proteins have secondary functions independent of their involvement in protein biosynthesis. A number of these proteins function as cell proliferation regulators and in some instances as inducers of cell death. Specifically, expression of human ribosomal protein L13a has been shown to induce apoptosis, presumably by arresting cell growth in the G2/M phase of the cell cycle. In addition, inhibition of expression of L13a induces apoptosis in target cells, suggesting that this protein is necessary for cell survival. Similar results have been obtained in the yeast Saccharomyces cerevisiae, where inactivation of the yeast homologues of L13a, rp22 and rp23, by homologous recombination results in severe growth retardation and death. In addition, a closely related ribosomal protein, L7, arrests cells in G1 and also induces apoptosis. Thus, it appears that a group of ribosomal proteins may function as cell cycle checkpoints and compose a new family of cell proliferation regulators.